This invention relates to a vehicle braking system comprising at least one brake control channel comprising a controller responsive to wheel speed signals from at least two wheel speed sensors, for sensing skid conditions of at least two wheels and for generating skid control instructions, a plurality of wheel brakes responsive to fluid pressure, hereinafter referred to as brake pressure, supplied thereto, a supply means, comprising a common supply valve, for supplying brake pressure to said brakes from a fluid pressure supply in accordance with a brake operating signal and a skid control means controlled by said skid control instructions for controlling the brake pressure in a plurality of cycles each of which comprises a pressure reduction phase and a pressure increase phase and may also comprise a pressure hold phase between the pressure reduction and increase phases.
For example, the channel or each channel may comprise two wheels at opposite ends of a common axle or may comprise two wheels at the same end of different axles. The wheels may be of any type, e.g., driven, steered or load carrying.
When a "split co-efficient of friction situation" arises, i.e., when one wheel of a channel system is engaged with a surface having a co-efficient of friction which is higher (higher friction surface) than the co-efficient of friction of a surface with which another wheel is engaged (lower friction surface), it has been known to select one of the wheels as the wheel which controls the operation of the skid control means. If the wheel engaged with the lower friction surface or a lower friction surface is selected (known as "select low mode") then, when the brakes are applied the skid control means is operated, in dependence on the controller detecting a skid condition of the wheel engaged with the lower friction surface, to cause the common supply valve to control brake pressure to reduce the braking effect on all the wheels of the channel. Where the channel comprises more than two wheels normally the skid control means is controlled by the wheel engaged with the lowest friction surface, i.e., the first wheel to skid. However, if desired, any surface other than the highest friction surface may be selected if desired and such surface is referred to herein as the lower friction surface.
In a select low mode the controller controls the brake pressure of the wheels in accordance with skid conditions of the wheel engaged with the lower friction surface. This causes a reduction in the braking effect of the wheel(s) engaged with the higher friction surface or each higher friction surface so that overall braking effect is reduced and the retardation is approximately equivalent to the wheels being engaged with the lower friction surface.
Alternatively, if the wheel engaged with a higher friction surface is selected (known as "select high mode"), then when the brakes are applied the skid control means is operated in dependence on the controller detecting a skid condition of the wheel engaged with the higher friction surface, to cause the common supply valve to control brake pressure to reduce the braking effect of the wheels.
When the channel comprises more than two wheels normally the skid control means is controlled by the wheel engaged with the highest friction surface, i.e., when all the wheels have skidded. However, if desired, any surface other than the lowest friction surface may be selected if desired and such surface is referred to herein as the higher friction surface.
In a select high mode the controller controls the brake pressure of the wheels in accordance with skid conditions of the wheel engaged with the higher friction surface. This causes the wheel(s) on the lower friction surface or each lower friction surface to continue to skid. In these circumstances, whilst the wheel(s) engaged with the lower friction surface or each lower friction surface contributes significantly to retardation of the vehicle, this wheel provides little stability, although by providing skid control to the wheel(s) engaged with the higher friction surface(s) retardation is maximized from the wheel(s) engaged with the higher friction surface(s) and overall stability is maintained.